In the exploration for hydrocarbon bearing formations in earth formations traversed by well bore, a logging tool is suspended in the well bore by means of an electrical armored cable. The cable is attached at one end to a well tool in a well bore and at an opposite end to a spooling winch on a logging truck at the earth's surface. The spooling winch is utilized to lower or raise the logging tool in the well bore so that the logging tool can obtain data relative to the earth formations adjacent to the logging tool. The well bore is usually filed with a logging mud or other control fluids and the logging is usually accomplished while pulling the tool from the bottom of the well bore towards the surface. For the data obtained relative to the earth formations to have any relevant meaning, it is necessary to correlate the obtained data to the depth of the obtained data from the earth surface. This is accomplished by measuring the length of cable extending from the earth surface to the logging tool.
Measurement of the cable length is usually accomplished by either a mechanical wheel system or a magnetic mark system or a combination of the systems. In the mechanical wheel system, a calibrated wheel tangentially contacts the cable at the earth surface and is rotated in response to cable movement. The rotation of the wheel is translatable into a function of the cable length which passes by the wheel. In the magnetic mark system, a cable is marked with magnetic marks at predetermined and defined intervals along the length of the entire cable. When a magnetic mark on the cable passes a magnetic mark detector at the surface, the mark is detected and the depth or the length of the cable is determined relative to the number of predetermined intervals counted.
There are number of factors which affect the actual length of a cable carrying a well tool in a well bore. For example, a cable has elastic stretch because of the weight of the cable and because of the weight of the well tool in the well bore. Thus, magnetic marks spaced at a typical spacing of 100 feet under a given tension are spaced apart more than 100 feet in borehole use because of the increased tension which causes elastic stretch in the cable in use.
A cable is a composite structure and has a permanent stretch characteristic. That is, the cable will elongate permanently by the application of tension until it is sufficiently "seasoned" to eliminate further significant permanent stretch. In addition, temperatures encountered in a well bore will produce an effect on the cable which cause stretch to occur while in a borehole.
The characteristics of cable stretch are well known for plastic and permanent stretch. Stretch tables are available for the various types of cable which will provide a determination of elastic stretch or permanent stretch for given conditions of tension. However, the logging operation is not a static condition. The well tool dynamically moves through a well bore fluid so that tension and the stretch length of a cable in the borehole are constantly changing and thus corrections made while the cable is moving are approximations and are not precise.
The measurement by the wheel at the surface can be in error because of slippage between the wheel and the cable. Wear of the wheel surface alters its calibration. Bearings and linkages tend to cause wheel torque and slippage. Thus, the depth measurement obtained from a wheel can be inaccurate.
With magnetic marks along the cable a number of problems can arise in that there can be extraneous false magnetic marks along the cable which can trigger an error in the depth measurement and the magnetic marks can deteriorate in strength so that they become unmeasurable or undetectable in operation.
Permanent or construdtion stretch in the cable can be a significant factor before the cable is "seasoned" so that any elastic stretch calculations can be insignificant until a sufficient number of trips of cable in the borehole stabilize the cable as to permanent stretch. This may require the operator to remeasure and place new magnetic marks on the cable after every job until the cable is permanently stretched.
The cables have an elastic stretch coefficient which is a predetermined value for a given type of cable but the cable manufacture can cause the elastic coefficient to vary from end to end on an individual cable and inaccuracy occurs because the existing stretch chart tables are based upon an average coefficient rather than an actual coefficient of the cable. In present systems there is no way to adjust for a different stretch coefficient for a cable.
The precise depth measurement for the data which is recorded on a well log as a function of depth is significant because it is customary to obtain several different logs at different times of a well bore and to correlate the data for interpretation purposes. Obviously, if the depth measurement for the data varies from run to run, no meaningful correlation can be accomplished. In addition, where different depth measurements for data result it is difficult to determine what the accurate depth is for the data. This is particularly significant where attempting reservoir analysis from spaced wells or correlating strata between wells.
In recent years the trend in completion practices has been to produce from thinner sections of oil or hydrocarbon bearing formations and, thus, if an operator is attempting to produce a four foot interval of sand at a depth of 8,000 feet, an incorrect depth measurement can result in a bad completion, increased cost and expense, and possibly loss of the well.
Heretofor, a number of systems have been proposed for depth measurement which include the following:
U.S. Pat. No. 3,753,294 issued Aug. 21, 1974 to Attali et al which utilizes a distributed capacitance value in a cable for correcting cable measurements.
U.S. Pat. No. 3,465,447 issued Sept. 9, 1969 to D. E. Bowers et al in which tension measurements are utilized to correct cable measurements.
U.S. Pat. No. 3,465,448 issued Sept. 9, 1969 to W. A. Whitfill, Jr. in which tension measurements are utilized to correct depth measurements.
U.S. Pat. No. 4,117,600 issued Oct. 3, 1978 to Guignard et all in which cable wire line movement measurements are made for correction of depth of measurements.
U.S. Pat. No. 3,490,150 issued Jan. 20, 1970 to W. A. Whitfill, Jr. in which correction signals are applied to the depth measurement system.
In a well bore, the weight or load of a cable and the weight or load of the well tool affect the stretch of the cable. Also, while moving through drilling mud or control fluids in a well bore, the load on the cable is affected by the cable speed and the borehole tool fluid relationship going in or out of a well bore which can increase or decrease tension and hence length of the cable.
As mentioned before, there are two different types of cable stretch, i.e. permanent elongation and elastic elongation. Permanent elongation, also known as constructional or plastic stretch, is the amount of stretch or increase in length in a cable which occurs over time and use until a cable is "seasoned". A seasoned cable is one in which irreversible elongation or stretch does not appreciably occur with use.
As an example of permanent stretch, a 7/16" diameter, 7 conductor cable with a length of 10,000 feet will permanently stretch about 50 feet with about 30 pulls in a well bore at 7200 pounds. With this same cable, it could be stabilized with 30 pulls in a well bore at 5000 pounds, however, the first time that the cable is tensioned to 6000 pounds there will be additional permanent stretch. In other words, permanent stretch can occur any time a cable is subjected to tension greater than the tension at which the cable is seasoned.
Elastic elongation or elastic stretch is commonly defined as a coefficient in the form of feet of stretch per 1000 feet of cable per 1000 pounds of tension (Ft./1000 ft./1000 lb). The typical coefficient of stretch value for a 7/16 inch diameter, 7 conductor cable is 0.85. These are the values that most stretch charts are based on. For practical purposes, a normal stretch coefficient is very close to 0.1 inch/100 Ft./100 lb. meaning that for every change in tension of 100 pounds, 100 feet of cable will lengthen (or shorten) 0.1 inch.
In using cables it is obvious that use will affect the cable length. Thus, a cable history of the number of runs and maximum tension is important to establish whether a cable is seasoned or not. Since the total amount of stretch is the sum of the permanent and the elastic stretch values, the number of times a cable has been tensioned and the tension values must be known in order to predict whether stretch in a cable is permanent and elastic stretch, or only elastic stretch.
As an example of how cable measurement errors occur, assume that 5000 feet of 7/16 inch, 7 conductor cable is magnetically marked at 100 feet intervals at 1000 pounds and is spooled on a truck reel at 3000 pounds except for the last two layers which are installed at 1000 pounds. If it is assumed that one layer is 750 feet of cable, at the end of this operation the wheel count should be different from the cable marker measurement by feet. This is because 13,500 feet of cable was tensioned at 3000 pounds and 1500 feet of cable was tensioned at 1000 pounds. The elastic stretch is determined by the following relationship. EQU stretch (whole count in feet)=0.85.times.13.5.times.(3-1)=22.95 ft.
In the above calculation, several sources of errors have not been considered and it is assumed that:
(1) the wheel measuring device is completely accurate; PA1 (2) no permanent stretch has occurred PA1 (3) the marking tension was exactly 1000 pounds PA1 (4) the spooling tension was exactly 3000 pounds.
The typical measuring wheel accuracy is 0.1 to 0.5% percent (A clean unworn wheel can approach 0.03%) and using 0.03% percent as average, the wheel can cause an additional error of .+-.15 feet in 15,000 feet. The other three errors can easily add up to .+-.15 feet if the cable history is unknown and tension control was difficult.